1. Field of the Invention
This invention is an optical fiber fabrication method and apparatus.
2. Description of the Prior Art
During the past decade significant advances have been made in developing the constituents of a commercially viable optical fiber communication system. Among these constituents are appropriate optical sources, detectors and transmission cables, and in more sophisticated designs, highly complex integrated optical circuitry. One of the earliest developments which allowed for the fabrication of a commercially practical system was the fabrication of a low loss optical fiber for use as the transmission medium. These fibers, made from very pure glass, and in certain embodiments coated with additional protective materials, can now be fabricated with losses as low as 0.5 db/km in the optical and near infrared region of the spectrum. While these fibers may be single mode or multimode, early commercial applications appear to lean toward the multimode variety. In such a multimode fiber, dispersive effects arising from the dependence of the effective velocity of the light on the particular mode in which the light is transmitted have been significantly minimized by fabricating the optical fiber with a radial gradation in index of refraction, which then compensates for the otherwise prevalent mode dispersion phenomenon.
Two major techniques are currently used in the fabrication of optical fibers. One involves the "soot" process described, for example, in U.S. Pat. Nos. 3,775,075 and 3,826,560 assigned to the Corning Glass Works and the other involves the MCVD process described in U.S. Patent Application Ser. No. 828,617 filed on Aug. 29, 1977, now U.S. Pat. No. 4,217,027, by J. B. Mac Chesney and P. B. O'Connor and assigned to Bell Telephone Laboratories, Incorporated. The soot process, as it is most commonly practiced, involves the production of glass precursor particulate material in a flame hydrolysis burner from glass precursor vapors, such as SiCl.sub.4 and oxygen, as well as from various additional glass precursor vapors, used for doping purposes, such as BCl.sub.3 and GeCl.sub.4. The particulate material, usually referred to as soot since it is formed in a flame, is then deposited on a solid cylindrical mandrel which may or may not be a part of the ultimate fiber. If the mandrel is not part of the ultimate fiber, it is removed after deposition. If a graded fiber is required the composition of the deposited material may be varied from layer to layer. Subsequent to deposition, the particulate material is consolidated into a glass by heating to appropriate temperatures in a carefully controlled environment. The resultant "optical fiber preform" is then drawn into an optical fiber.
In the MCVD process, the above-described glass precursor vapors are flowed through a glass tube which is heated. In this environment, the glass precursor vapors react homogeneously to form particulate material. This particulate material is distinguished from the above-described soot in that it is not formed in a flame but is rather formed in a flameless heated environment. Numerous layers are deposited and immediately consolidated to a glass on the inside wall of the tube. Layers of varying composition may be formed if a graded fiber is ultimately required. Subsequent to deposition the structure is collapsed to a solid "optical fiber preform" which is then drawn into an optical fiber.
From this description of the prevalent optical fiber fabrication processes, it may be readily seen that an essential element in both processes involves the delivery of glass precursor vapors to the fabrication apparatus. The rate of fabrication is dependent in part on the rate of such delivery. In addition, the ability to fabricate layers of uniform predetermined composition depends on the ability to deliver glass precursor vapors at specifically controlled concentrations. Heretofore, exemplary delivery systems have included firstly a receptacle containing glass precursors in liquid form, e.g., SiCl.sub.4, GeCl.sub.4, and POCl.sub.3. An appropriate carrier gas such as oxygen is then bubbled through the liquid to vaporize a portion of it and transport it to the fabrication apparatus. In order to be able to determine, readily and quantitatively, the amount of vapor delivered to the fabrication apparatus, the flow of carrier gas must be sufficiently slow so that it becomes saturated with the vapor in the course of passing through the appropriate liquid. Clearly, such a process is limited, by the relatively slow process, in the amount of readily calculable material that it can deliver per unit time.
Recent attempts to increase the amount of vapor capable of being delivered, while retaining the ability to accurately calculate the amount of material so delivered, have centered around increasing the rate of vaporization during the course of bubbling the carrier gas through the appropriate precursor liquid. As mentioned above, during the bubbling process the liquid vaporizes and saturates the carrier gas stream. Since this is an equilibrium process, the bubbling must be sufficiently slow so that the carrier gas will indeed become saturated, thereby permitting simple calculation of the amount of vapor delivered. Improvements on this technique center about increasing the rate at which the vapor enters the carrier gas stream and saturates it. For example, some practitioners heat the liquid through which the carrier gas stream is bubbled so as to speed up this process. Other techniques center about increasing the surface area of the liquid which is contacted by the carrier gas stream thereby increasing the amount of vaporization. Although all of these techniques provide some improvement in the rate at which the vapor can be delivered while still being capable of easy calculation, advantages will accrue if the rate of delivery can be reproducibly increased further while remaining under accurate control.